1. Field of the Invention:
This invention relates to attenuating devices, and more particularly, to devices which utilize a YIG material to provide frequency selective attenuation of microwave signals above a preselected threshold power level.
2. Description of Related Art:
Frequency selective limiting (FSL) or attenuating devices which utilize a yttrium-iron-garnet (YIG) material have the property of being able to attenuate higher power level signals while simultaneously allowing lower power level signals, separated by only a small frequency offset from the higher level signals, to pass with relatively low loss. YIG-based FSLs are capable of limiting or attenuating across more than an octave bandwidth in the 2-8 GHz range. Higher power level (above-threshold) signals within a selectivity bandwidth will be attenuated without requiring tuning of the FSL. Lower power level (below-threshold) signals, separated from the higher power level signals by more than a few spinwave linewidths, will pass through the FSL without experiencing any greater loss than if the higher power level signals were not present. For an attenuating device based on YIG, this selectivity bandwidth is on the order of between 20-50 MHz.
YIG-based FSLs have many applications in microwave signal systems. One such application is illustrated in FIG. 1. The microwave signal system 10 includes an antenna 12 for collecting and passing microwave RF signals, a YIG-based FSL device 16 and a broadband receiver 14 (hereafter sometimes referred to as receiver 14). Microwave signal processing equipment 18 is responsive to the output of the receiver 14. The microwave signal processing equipment 18 is of a type presently known in the art and will not be further described.
FSL 16 is utilized to increase the dynamic range over which microwave signals collected by the antenna 12 can be accepted by the receiver 14. Because known receivers such as broadband receiver 14 generally have a dynamic range of approximately 35 dB, and signals of interest arriving at antenna 12 may have a dynamic range of, for example, 85 dB, it can be readily appreciated that a power mismatch is created within system 10. The mismatch is corrected by utilizing the FSL device 16 which may be designed to provide a dynamic range of about 50 dB, approximately mid-point between the signal level at the antenna 12 and the dynamic range of the receiver 14.
FSL 16 is designed to provide that the ratio of power out to power in (P.sub.out /P.sub.in), below a predetermined threshold value of TP.sub.in, is substantially linear. As the value of input power P.sub.in seen by FSL 16 increases above the predetermined threshold value of TP.sub.in, the ratio of P.sub.out /P.sub.in becomes smaller. Stated in another manner, FSL device 16 operates to attenuate an above threshold, high power input microwave signal having a large dynamic range to provide an output signal having a smaller dynamic range.
A YIG-based frequency selective limiting (FSL) device 20 discussed in U.S. Pat. No. 4,845,439; entitled "Frequency Selective Limiting Device," in the name of Steven N. Stitzer et al. and assigned to Westinghouse Electric Company the assignee of the present invention is illustrated in FIGS. 2A and 2B. Attenuation in the FSL 20 is proportional to the volume of YIG material in layers 22 and 24 which is coupled to the RF magnetic-field 26 generated by the signal-carrying conductor 28. While the configuration and positioning of the YIG layers 22 and 24 relative to the signal-carrying conductor 28 results in satisfactory coupling of the RF magnetic field 26 with YIG material in layers 22 and 24, the configuration of the FSL 20 is difficult and expensive to fabricate.
According to current manufacturing procedures a narrow signal-carrying conductor 28 is sandwiched between two thin layers 22 and 24 of single crystal yttrium-iron-garnet (YIG). The YIG layers are typically about 0.002 to 0.005 inch thick. Effective limiting requires a strong coupling of RF magnetic-field 26 with the YIG for a given RF power level. Accordingly, in order to confine the magnetic-field 26 within the YIG layers 22 and 24, the sandwich is surrounded by a ground plane 29. The arrangement of FIGS. 2A and 2B ensures that substantially all the RF field lines 26 pass through the YIG layers 22 and 24.
The device 20 is currently made by epitaxially forming separatly, each YIG layer as a thin layer of single crystal YIG on a gadolinium-gallium-garnet (GGG) substrate (not shown). Each layer of GGG is then removed by a grinding step. It is also necessary to use a separate metalized GGG substrate 30 as a device support. The metalized surface 32 separates the GGG substrate 30 from the YIG material of layer 24 and completing the group plan. The current process is expensive and time consuming.
In order to achieve a greater level of attenuation, a plurality of individually formed separate devices 20 may be connected in series as shown in FIG. 3. The FSL devices 20 are positioned in parallel with a plurality of D.C. biasing magnets 34 located therebetween which provide a transverse biasing field 36. Each FSL 20 is separately manufactured.